The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
Free tracker included with every download this week
How Do Mounjaro Injections Work?
Mounjaro is a prescription-only weight management injection that works differently from other weight loss medications available in the UK. It's a dual GIP and GLP-1 receptor agonist — meaning it activates two natural hormone pathways in your body to help regulate appetite and support weight loss. This dual action is what sets Mounjaro apart from single-mechanism injections like Wegovy. Understanding how Mounjaro works can help you decide whether it might be suitable for you, alongside a reduced-calorie diet and increased physical activity. Always discuss eligibility and suitability with your GP or a licensed prescriber.
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Calculate Your BMI Instantly
Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
The Dual Mechanism: GIP and GLP-1
Mounjaro contains tirzepatide, which activates two hormone pathways simultaneously:
- GLP-1 (glucagon-like peptide-1) — a hormone that signals fullness to your brain, reduces appetite, and slows how quickly food leaves your stomach
- GIP (glucose-dependent insulinotropic polypeptide) — another natural hormone that helps regulate blood sugar and may enhance the appetite-suppressing effect
By activating both pathways, Mounjaro may help you feel fuller after smaller meals and reduce cravings more effectively than single-mechanism injections. The dose starts low (2.5 mg) and increases gradually every 4 weeks until you reach your target dose (maximum 15 mg weekly). This dose titration approach helps your body adjust and reduces side effects. In clinical trials, Mounjaro helped participants lose an average of 15–21% of their body weight — though individual results vary significantly.
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
What Happens in Your Body
Once you inject Mounjaro under the skin (stomach, thigh, or upper arm), the medication enters your bloodstream and activates GLP-1 and GIP receptors throughout your digestive system and brain. This triggers several effects: your appetite decreases, you feel satisfied with smaller portions, and your stomach empties more slowly — so you stay fuller longer. These effects help create a calorie deficit without extreme hunger.
Mounjaro is prescription-only and requires a clinical assessment to confirm eligibility. You can access it through NHS specialist weight management services (with a referral from your GP) or through a private online prescription service following a clinical consultation. Use our GLP-1 weight loss calculator to explore projected outcomes at different doses, and our BMI calculator to check where you currently stand against NHS healthy weight ranges.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Is Mounjaro the same as insulin?
No. Mounjaro is not insulin — it's a GLP-1 and GIP receptor agonist. It doesn't inject insulin into your body; instead, it activates natural hormone pathways that help regulate appetite and blood sugar. It can be used by people with or without type 2 diabetes.
How long does it take for Mounjaro to work?
Most people notice appetite reduction within the first few days to weeks. Significant weight loss typically becomes visible after 4–8 weeks. Full effects are usually reached after 16–20 weeks when you've reached your target dose. Results vary between individuals.
Can I use Mounjaro if I'm not diabetic?
Yes. Mounjaro is licensed for weight management in UK adults with a BMI of 30 or above, or 27–29 with weight-related health conditions — regardless of diabetes status. Your prescriber will assess your individual eligibility.
How does Mounjaro compare to Wegovy?
Mounjaro uses dual GIP and GLP-1 action, whilst Wegovy uses GLP-1 only. Mounjaro doses range from 2.5–15 mg weekly; Wegovy ranges from 0.25–2.4 mg weekly. Clinical trial data suggests Mounjaro may produce slightly greater average weight loss, but individual results vary — the best option depends on your personal circumstances and prescriber recommendation.
What are the most common side effects of Mounjaro?
Very common side effects include nausea, diarrhoea, constipation, and stomach pain. These often improve as your body adjusts. Less common side effects include injection site reactions, fatigue, and hair loss. Seek urgent medical attention for severe stomach pain, serious allergic reactions, or vision changes.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.